Rough idle/ misfire on an 85

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KINGJOHN808
Posts: 127
Joined: Sat Feb 16, 2019 2:27 am
Location: Ewa Kai

Re: Rough idle/ misfire on an 85

Post by KINGJOHN808 »

TimU wrote: Wed Apr 27, 2022 8:44 am Aloha John,

Are you getting spark from the coil so that you can identify which side of the spark-chain to start troubleshooting?

Checks:
- Loose engine earth straps (plural) - Commonly the one at the top-rear of the valve cover can get loose.
- Loose Battery terminals.
- Worn distributor cap & rotor button. Could also be gunked-up from an oil leak behind the rotor boss.
- Main Relay - One pin 87 can go faulty.
- Crankshaft Position Sensor - A faulty sensor will causes a loss of spark.
- Flywheel Speed Sensor - A faulty sensor will allow the car to start, but it will die soon after.
- Coolant Temp Sensor (blue head) - A faulty/disconnected CTS will stop the engine (unsure about spark).
- OBC Relay (under the dash) - A faulty OBC relay can cut power to the main relay.
- Cracked solder joint in the ECU

If you haven't already seen it, this article is an excellent troubleshooting guide:
https://hpsimotorsports.com/blogs/tech- ... ch-article
(PDF attached for posterity)


Bosch Motronic Fuel Injection Tech Article - HPSI Motorsport.pdf
Aloha TimU

Got my replacement CPS and Speed Sensor today. Installed and car still wouldn’t crank/stay running. Went into troubleshooting mode. Checked, fuel, coil, plugs, wires, injector input, battery voltage still couldn’t figure out why it wouldn’t start but crank.

Starting thinking about things I had done/installed since the last time it ran. Checked fuel pump, OBC and display unit, tail lights and head lights, radio, amp, etc. finally broke out the ETM and started tracing and probing. Everything pointed to a possible bad relay. Started swapping them out one at a time and still no luck.

Decided to look up known “good” replacement units and realized that I mixed up a 4 prong and a 5 prong up when I removed the relays to clean them some time ago. Funny thing is the car started, ran like crap and then shut off.

So, long story short; I spent the better part of 3 hours trying to figure out what the problem was only to realize I had introduced this problem by putting the relays in the wrong locations! After starting the car several times, called it a night and came in the house around 1130 at night.

I’m in the process of labeling relays and electrical fittings just in case I have another issue like this. One thing for sure, the car will not run with a faulty CPS.
Ewa Kai, Hawai’i
1984 633CSi
1983 633CSi (wreckers)

“Aloha and have a great day!”
TimU
Posts: 280
Joined: Tue Dec 14, 2021 2:16 am
Location: Sydney

Re: Rough idle/ misfire on an 85

Post by TimU »

Well done, and very easy to inadvertently do!

In the IT biz, we call that an "ID 10 T" error :lol:
04/1986 635csi Euro spec
Australian Delivered, RHD
songzunhuang
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Re: Rough idle/ misfire on an 85

Post by songzunhuang »

Well, I just discovered this thread and am following it with interest. My car is running horribly and having various start issues. After doing tons of diagnostics and parts swapping, I finally had my car towed to the mechanics. I just didn't have time (or the weather) to mess with it and I wanted to get the car fixed.

Also, I have made the mistake of swapping the wrong relays, except mine was related to the headlights. It drove me crazy for a while. It' a mistake I will not make again.

I'll let folks know what I discover after paying dearly to the mechanic.
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Song Huang
1984 633CSi
Last 7 of VIN: 6997383
Jackie&Gracie
Posts: 6
Joined: Fri Nov 11, 2022 7:51 pm
Location: York, PA

Re: Rough idle/ misfire on an 85

Post by Jackie&Gracie »

Certainly late getting here and perhaps the rough running has been resolved. I hope so. If not, here is my input -

First - knowing mileage would be beneficial. Gives some reference to things that might be due for replacement, etc.

Second - the BIG one in my mind - “Looked at vacuum hoses, looked fine” Or something to that effect. LOOKING at them tells you very little. At least pinch them to see if they are hard (replace those). Wiggle them to see if they slip off easy(replace those too). Look at them to see if they look original (38 year old rubber hoses? And maybe they are still good? Really?)

This car DEPENDS on its vacuum system to run. There are hoses everywhere and they all get old and hard. Then they start to leak. The result would be a slow, incremental loss of engine performance.

Why am I pointing at this? (I too am very new here). My ‘88 635 was not running as well as I thought it should. It has 68k original miles on it. It runs but “performance car” isn’t what came to mind when I drove it. I started by checking the easy, obvious stuff. Wiggle hoses under the hood. New clamp here, shorten and another clamp there. I found a LOT of loose ends and EVERY TIME I fixed something it ran a little better. A notable improvement was a new fuel pressure regulator and tighter hoses to and from that,
Another red flag for me suggesting the vacuum system is when the OP said “pulled the dipstick and the engine quits”. The logic kind of goes like this - with vacuum leaks it runs poor, more leaks and it runs worse. Bad leaks and it won’t run at all. Pulling the dipstick was the straw that killed it.
As an example, There is a hose (several actually) under the intake manifold. Can’t see it. Feels to be about 1/2” ID. It can be easily slipped off the upward pointing metal tube connection that is also invisible to casual observation. When off the car won’t start at all. When on it runs fine. Found that by accident. Retraced my steps, put it back in place and I am driving it now. It is on my to-do list.

I have bought a kit to replace ALL the vacuum hoses. Good common sense in my mind. Someone before me did replace radiator hoses and a few fuel hoses. The easy and obvious stuff. This is where mileage and age is a consideration. Mine has that 68k on the odometer. Many ORIGINAL hoses under the hood. That mileage OR that age - they need replaced.

My 635? Every time I fix a hose it acts younger, happier and more spirited. And I trust it to be more reliable.

Try the easy stuff.
songzunhuang
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Re: Rough idle/ misfire on an 85

Post by songzunhuang »

I wanted to close the loop on the issue with my car that caused it to get towed to the shop.
Turns out that I had a broken connection from harness for the temp sensor mounted in the thermostat housing cover.

Way back in 1986 BMW had a technical bulletin where it had dealers install an in-line resistor to the harness connected to the thermo time sensor. The trick was the resistor was soldered and hidden under the rubber boot at the harness. Over time, the resistor crumbles and the connection is broken. You can't see it (under the rubber boot) so that's what makes it tricky.

Since there was no signal, the ECU just kept feeding more fuel (thinking it's cold) or shutting off the fuel pump completely.
Drove me crazy for a long time. Since the 5 minute fix (remove the resistor and fix the gap in the wire), I've driven over 210 miles and everything is smooth again.

This wasn't done to all cars so check to see if yours is affected. There's a post with a link to this bulletin if you search the forum.
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Song Huang
1984 633CSi
Last 7 of VIN: 6997383
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